This invention relates to a position detecting method and a position detecting system. Also, the invention concerns an exposure apparatus, a device manufacturing method, a semiconductor manufacturing factory, and/or a maintenance method for an exposure apparatus, using such a position detecting system. In another aspect, the invention is directed to an interference microscope for measuring the position of a pattern on an object, to be observed through an imaging optical system, and for detecting the position of that object on the basis of it.
For example, the invention is applicable to an alignment detecting system in an exposure apparatus for semiconductor manufacture, or an overlay inspecting system to be used for inspection of distortion or any performance such as alignment precision, in an exposure apparatus.
Projection exposure apparatuses for semiconductor manufacture are required to have a performance of projecting and printing a circuit pattern formed on the surface of a reticle onto the surface of a wafer with a high resolving power, to meet the increasing density of integrated circuits. The projection resolving power for a circuit pattern can be improved, for example, by enlarging the numerical aperture (NA) of a projection optical system while keeping the wavelength of exposure light unchanged. Alternatively, it can be done by shortening the wavelength of exposure light, such as changing light from g-line to i-line, from i-line to excimer laser emission wavelength, or to F2 laser emission wavelength or SOR light.
On the other hand, there are requirements for higher precision alignment of a reticle (having an electronic circuit pattern) and a wafer, to meet further miniaturization of circuit patterns. The required precision is generally one-third or less of the circuit pattern. For a 1 gigabit DRAM with a 0.18 micron rule circuit pattern, for example, an overlay precision of 60 nm or less (alignment over the whole exposure region) is required. Further, as for an overlay inspection system for measuring this overlay precision, a precision of one-tenth of the overlay precision is required. Thus, a precision of 6 nm or less is necessary, for a 1 gigabit DRAM.
There is a TIS correction method for reducing the influence of a TIS (Tool Induced Shift), which is a detection system factor among measurement error producing factors, to attain high precision measurement.
Referring to FIG. 1, the TIS correction method will be explained. In FIG. 1, as an example, a surface step (level difference) is produced on a silicon wafer by an etching process, and, after alignment with that pattern, the relative relationship with respect to a resist image pattern defined by exposure and development is measured. In the TIS correction method, the measurement is carried out twice. The second measurement is made with the wafer being rotated 180 degrees, as compared with the first measurement. Thus, the result in the first measurement is called a xe2x80x9c0-deg. measured valuexe2x80x9d, and the result in the second measurement is called a xe2x80x9c180-deg. measured valuexe2x80x9d. In the TIS correction method, a value (xcex94TIS correction) which is obtainable by subtracting the 180-deg. measured value from the 0-deg. measured value and then by bisecting the remainder, is taken as a measured value. With this procedure, the error due to the detection system factor is reduced to attain high precision measurement. Here, the value obtained by adding the 0-deg. measured value and the 180-deg. measured value and then by bisecting the sum, is called a xe2x80x9cTISxe2x80x9d.
Most of the overlay inspection systems or alignment detection systems, currently available, use a bright-field image processing method. FIG. 2 shows a known example of an overlay inspection system.
In the inspection system of FIG. 2, exclusive marks 2 and 3 are formed on a wafer 1. Images of these marks are formed through an optical system upon an image pickup device 14 such as a CCD, and the position is detected by processing an electric signal from the CCD.
The imaging performance most required in this optical system is the symmetry of image. If there is something in the optical system that deteriorates the image symmetry, a TIS is produced.
These types of detecting systems use a high optical magnification such as 100x, for example, and in most cases, a region close to the optical axis is used. Therefore, the major factor causing degradation of the image symmetry is not abaxial aberrations but non-uniformness of the illumination system and eccentric coma aberration close to the optical axis of the optical system.
In recent semiconductor processes, flattening has been advanced, and a CMP (Chemical Mechanical Polishing) process is carried out at plural steps.
However, the flattening technology raises a critical problem to alignment or overlay inspection machines. As a result of the flattening process, the level difference (surface step) of a mark to be used for the detection is eliminated. In the bright-field image processing method which is used most prevalently and with a good precision, the contrast of a mark image to be used for the measurement becomes very poor, which directly leads to deterioration of the detection precision.
As a measure for such a problem, a phase difference detecting method has been proposed. However, this method needs a phase plate in a portion of the optical system, and this is a factor for producing a TIS as described above. Therefore, while the contrast may be better, a good precision may not be attainable. Particularly, since the phase plate should be made mountable/demountable into and out of the optical system so as to allow coexistence with an ordinary bright-field system, this creates a factor for a large TIS.
An interferometer detecting method may be a detecting method not sensitive to a TIS. An example will now be described, in conjunction with FIG. 3.
First, a conventional overlay inspecting system of FIG. 2 will be described, and second, the interferometer detecting system shown in FIG. 3 will be described.
In FIG. 2, an etching pattern mark 2 is produced on a silicon etching wafer 1, through a lithographic process, a development process and an etching process. Then, upon the etching pattern mark 2, a resist pattern mark 3 is formed through a lithographic process and a development process. In an overlay inspecting system, the relative positional relation between these two marks 2 and 3 is just going to be measured. To this end, the marks 2 and 3 are illuminated with light 6 emitted from a halogen lamp 5. More specifically, the light 6 from the lamp goes through a fiber 7 and an illumination system 8, and S-polarized light is reflected by a polarization beam splitter 9. Thereafter, the light passes through a relay 12 and via a mirror 4, and then through a quarter waveplate 10 and an objective lens 11, to illuminate the marks 2 and 3.
Reflected light from the marks 2 and 3 passes backwardly through the objective lens 11, the quarter waveplate 10 and the relay 12. Since the polarization direction is P-polarization, the light is transmitted through the polarization beam splitter 9. Then, by means of an erector 13, an image is formed on the image pickup surface of a CCD camera 14. The observation image thus formed on the image pickup surface is photoelectrically converted by the CCD camera 14, and a corresponding signal is applied to a computer (calculating means) 15 through a line. The computer 15 performs image processing to the received signal, and it detects the relative positional relationship between the two marks.
Here, the polarization beam splitter 9 and the quarter waveplate 10 are used for the sake of efficient use of the light quantity. If the light source has a large power or the object to be observed has a large reflection factor such that the loss of light quantity can be disregarded, use of a polarization beam splitter or a quarter waveplate may be omitted. A half mirror may be used, for example, in place of it.
The wafer 1 is placed on a wafer chuck (not shown). The wafer chuck is mounted on a xcex8-Z stage (driving means, not shown), and it functions to attract the wafer 1 toward the chuck surface, so as to prevent shift of the wafer 1 position against various vibrations. The xcex8-Z stage is mounted on a tilt stage (not shown), and it functions to move the wafer 1 upwardly and downwardly, along a focus direction (optical axis direction of the optical system).
Now, an example of an interferometer detecting method will be described, with reference to FIG. 3.
The inspecting system of FIG. 3 uses a light source which may be a Hexe2x80x94Ne laser 31, for example. Except that the light source is different, the structure for forming an image of the wafer 1 upon the image pickup surface of a CCD camera 14 is essentially the same as that of the FIG. 2 example. However, in this example, a quarter waveplate 21 is inserted between the polarization beam splitter 9 and the erector 13, for strengthening the coherence with reference light effective to provide an observation image as an interference image.
Further, since the light source is coherent light using the Hexe2x80x94Ne laser 31, if it is used directly for the illumination, a speckle will be produced on the wafer 1. In order to avoid this, the light has to be made incoherent. This can be done in various ways. In the example of FIG. 3, a rotary diffusion plate is disposed between the light source (Hexe2x80x94Ne laser) 31 and the fiber 7, to move and thus to average the speckle within a time period in which it is taken in by the CCD camera 14 as an image.
The reference light for providing an observation image as an interference image, will be explained.
The light 6 emitted from the laser 31 passes through the fiber 7 and the illumination system 8, and P-polarized light is transmitted through the polarization beam splitter 9. The P-polarized light is then transformed into circularly polarized light by the quarter waveplate 22, and it is reflected by a mirror 23.
Here, the mirror 23 is disposed at a plane optically conjugate with the wafer 1 to be observed. The reflected light again passes through the quarter waveplate 22, and it is transformed into S-polarized light. It is now reflected by the polarization beam splitter 9, and, after passing through the quarter waveplate 21, it is projected by the erector 13 on the image pickup surface of the CCD camera 14, as reference light.
By the reflection light from the wafer 1 and the reference light, described above, an interference image is produced.
An interference image can be produced in this manner, and this image has better contrast as compared with an image formed by conventional bright-field illumination.
As the focus of the wafer 1 changes, the interference condition also changes. It causes changes in the bright and dark portions of the interference fringe. FIG. 5 shows a case where an outside region 42 of an alignment mark is dark, under the interference condition. FIG. 4 shows a case where, as compared with FIG. 5, an outside region 42 of an alignment mark is bright, under the interference condition.
In consideration of this, by taking a difference between the interference images at two different focuses, an image with double contrast can be produced.
The above-described two focuses are those for which the difference in the output of the CCD camera 14 between the outside regions 42 of the alignment marks 41 in FIGS. 4 and 5 (i.e., the difference in image intensity between them) becomes largest and those having opposite signs.
For example, it is assumed that the level difference (step difference) of the alignment mark 41 is equal to one-fourth ({fraction (xcex/4)}) of the illumination wavelength xcex (633 nm in the case of a Hexe2x80x94Ne laser). In this case, if the focus is changed to a bright condition of interference where the optical path difference between the outside region 42 of the mark and the reference light becomes equal to a multiple, by an integer, of the illumination wavelength xcex, then, as shown in FIG. 4, the interference image in the outside region 42 of the mark has a strongest intensity. On the other hand, as regards the alignment mark 41, since the surface step difference is {fraction (xcex/4)}, the intensity becomes lowest. Here, if the reflection intensity of the reference light and the reflection intensity from the wafer are equal to each other, the visibility (sharpness) of the interference fringe becomes higher, and therefore the intensity is completely eliminated.
Next, if the focus is changed by {fraction (xcex/4)} from the preceding focus, then, as shown in FIG. 5, a dark condition of interference results where the optical path difference between the reference light and the outside region 42 of the mark corresponds to a multiple, by an integer, of the illumination wavelength xcex plus {fraction (xcex/4)}. Thus, the interference image in the outside 42 of the mark has a lowest intensity. As regards the alignment mark 41, on the other hand, since the level difference is {fraction (xcex/4)}, the intensity becomes highest.
FIG. 6 shows signals in these cases, obtained by photoelectric conversion. In the FIG. 5 signal, the alignment mark portion 41 is high, whereas it is low in the FIG. 4 signal.
Thus, by taking the difference between these two signals, a signal having a contrast two times higher than an interference image signal based on a single focus, is obtainable.
When the system to be used in conventional bright-field illumination which does not need reference light, a light blocking plate (shutter 24) shown in FIG. 3 may be inserted. This is similar to the structure of FIG. 2.
With this procedure, even for a CMP wafer having a small surface level difference, a high-contrast signal can be detected.
However, it has been found that the reflection factor of a wafer changes largely in dependence upon the process, and that the contrast of an alignment signal (interference signal) changes with the change in reflection factor.
This is a change in contrast of a fringe, which is called the visibility of interference fringe as described hereinbefore. When the reflection light from an object to be measured and the reference light have the same amplitude upon the interference observation surface and upon the CCD camera surface, in an image processing system, the interference fringe has a highest visibility. As the difference in amplitude becomes larger, the visibility of the interference fringe becomes lower. Thus, in various wafer processes, an optical image of stable and high contrast and a signal of high contrast are not obtainable.
It is an object of the present invention to provide a position detaching method and a position detecting system, by which, in regard to a subject of observation in various processes such as semiconductor processes, an optical image signal of high contrast can be obtained stably regardless of a change in reflection factor, and, particularly, by which stable and high precision position detection is enabled even in a process for a low surface level difference such as CMP.
It is another object of the present invention to provide a semiconductor manufacturing exposure apparatus in which the position detecting system such as described above is used as an alignment detecting system for the exposure apparatus or as an overlay inspecting system for inspection of the performance such as distortion or alignment precision, for example.
It is a further object of the present invention to provide a device manufacturing method or a semiconductor manufacturing factory, using such an exposure apparatus, or a maintenance method for such an exposure apparatus.
It is yet a further object of the present invention to accomplish high precision position detection by use of an inexpensive and compact system in which factors for deterioration of the footprint of an optical system member or factors for enlargement in size of the optical system member are excluded.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.